Results from a Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Climate Model

This animation shows the annual mean surface air temperature difference from the long-term average resulting from changes in the thermohaline circulation. Red and orange colors show regions of warming, blue shows cooling, and green indicates little temperature change.

Animation by New Media Studios

During the first 100 years of the simulation, an external source of freshwater is added to the North Atlantic between 50°N and 70°N at a rate of 0.1 Sverdrup (1 Sv = 10⁶ m³/s). This results in a reduction in the thermohaline circulation and cooling southeast of Greenland (dark blue). The cool region is most clearly developed in years 50 to 100. Following the end of freshwater addition at year 100, temperatures southeast of Greenland return to their original values within a few decades. By year 160, the cooling has disappeared completely, as indicated by yellow and green colors. Throughout the simulation, small fluctuations in temperature around the world are the product of noise in the climate system.

Conclusions

1. The results of this simulation support the hypothesis that freshwater additions to the North Atlantic cool climate over Europe and eastern North America.
2. The simulated cooling is smaller in size and area than some of the climate changes recorded by paleoclimate proxies. Some possible explanations for the mismatch are that past floods contained more freshwater than was added to the model, and that the model simulations began with present-day rather than past climate conditions.
3. In the modeling experiment, both the cooling and subsequent warming happen rapidly. The rapid cooling is forced by the sudden onset of freshwater addition, whereas the warming is caused by the dissipation of freshwater by mixing.